1. Field of the Invention
The present invention relates to an organic electroluminescence device (hereinafter referred to as an organic EL device) and a lighting equipment and a display device which are prepared by using the same.
2. Description of the Related Art
An organic electroluminescence device (hereinafter referred to as an organic EL device) is a light-emitting device having a constitution in which a light-emitting layer containing a luminescent compound is interposed between a cathode and an anode, wherein a hole injected from an anode and an electron injected from a cathode are recombined in the light-emitting layer by applying an electric field, whereby an exciton is formed, and light (fluorescence and phosphorescence) discharged when the above exciton is deactivated is used. Also, an organic EL device is a solid state device in which a space between electrodes is constituted by a film of an organic material having a thickness of only submicron and can emit light at a voltage of several voltages to several ten voltages, and therefore it is expected to be used for flat display and illumination of a next generation.
An organic EL device making use of phosphorescence emitted in an excited triplet state is reported from Princeton University as the development of an organic EL device toward practical application (refer to, for example, M. A. Baldo et al., Nature, vol. 395, p. 151 to 154 (1998)), and since then, material emitting phosphorescence at room temperature have come to be actively researched (refer to, for example, U.S. Pat. No. 6,097,147 and M. A. Baldo et al., Nature, vol. 403, No. 17, p. 750 to 753 (2000)).
Further, organic EL devices operated by making use of phosphorescence emission make it possible to achieve a luminous efficiency which is larger in principle by about four times than those of conventional organic EL devices operated by making use of fluorescence emission, and therefore a layer structure of a light emitting device and electrodes including the development of the materials for the above organic EL devices have been researched and developed all over the world. For example, many compounds including principally heavy metal complexes such as iridium complexes and the like have been synthesized and investigated (refer to, for example, S. Lamansky et al., J. Am. Chem. Soc., vol. 123, p. 4304 (2001)).
As described above, a phosphorescence emitting system has a very high potential, but a method for controlling a position of an emission center, especially recombining a hole and an electron in an inside of a light-emitting layer to stabilize emission is an important technical problem in terms of enhancing an efficiency and a life of the device.
In recent years, known well are devices of a multilayer lamination type comprising a hole-transporting layer positioned at an anode side of a light-emitting layer so that it is adjacent to the light-emitting layer and an electron-transporting layer positioned at a cathode side of the light-emitting layer (refer to, for example, JP-A 2005-112765). Also, a mixed layer formed by using a host compound and a phosphorescent compound as a dopant is used in a light-emitting layer in many cases.
On the other hand, materials which have a high carrier-transporting property and are thermally and electrically stable are desired from the viewpoint of the suited materials. Particularly in making use of blue phosphorescence emission, a blue phosphorescent compound itself has a high triplet excitation energy (T1), and therefore it is strongly desired to develop applicable peripheral materials and precisely control an emission center.
FIrpi is known as a typical blue phosphorescent compound, and it is shortened in a wavelength by substituting phenylpyridine of a principal ligand with fluorine and using picolinic acid as a subsidiary ligand. The above dopants provide highly efficient devices by combining carbazole derivatives and triarylsilanes as host compounds, but an emission life of the devices is deteriorated to a large extent, and therefore a trade-off thereof is required to be improved.
In recent years, metal complexes having specific ligands are disclosed as blue phosphorescent compounds having a high potentiality in U.S. Patent Publication No. 2011/0057559. Further, dibenzofuran and dibenzothiophene derivatives are proposed as host compounds used together with blue phosphorescent compounds in U.S. Patent Publication No. 2011/0057559. Organic EL devices obtained by using the host compounds described in U.S. Patent Publication No. 2011/0057559 are improved in a luminous efficiency and an emission life, but they are not yet satisfactory in terms of an operating voltage (reducing an operating voltage or decreasing a rise in a voltage in operation). Also, the luminous efficiency and the emission life have to be further improved.
An ageing stability is required to organic EL devices from the viewpoint of application to display and illumination.
For example, it is described in U.S. Patent Publication No. 2011/0057559 that anions or anion radicals are improved in a stability by using specific dibenzofuran and dibenzothiophene in combination with blue phosphorescent compounds and that the devices are improved in a life and a stability. In a light-emitting layer of an organic electroluminescence device comprising a dopant and a host compound, the luminous dopant is uniformly dispersed preferably in the host compound due to the problems of a transporting property of the carrier, concentration quenching caused by aggregation of the luminous dopant and quenching brought about by an interaction between the excitons. However, the states of the dopant and the host compound are changed in the light-emitting layer, for example, when the devices are caused to emit light for a long time or left standing under high temperature and high humidity, and as a result thereof, the device is likely to bring about such a degradation in performances as a rise in an operating voltage and a reduction in a luminous brightness. The viewpoint of the stability against such a change with the passage of time as described above is not described in U.S. Patent Publication No. 2011/0057559, and it has come to be apparent that the devices are insufficiently researched and have to be improved further more.
On the other hand, a wet method (called as well a wet process) is expected to a large extent due to requirements to the large area, the low production cost and the high productivity. Since a film can be formed at a lower temperature in the wet process than in forming a film by a vacuum process, the organic layer of a lower layer can be reduced in damage, and the wet process is expected to a large extent from the viewpoint of improving the luminous efficiency and the device life.
However, in organic EL devices operated by making use of blue phosphorescence, particularly a film-forming property of a host material contained in a light-emitting layer and an electron-transporting material laminated on a light-emitting layer, a solubility thereof in a solvent used for coating them, and the like are problems for preparing the devices by the wet process. Accordingly, host materials and electron-transporting materials which are known at present are still unsatisfactory in terms of a solubility in a solvent, a solution stability, an operating voltage and the like from the viewpoint of practical use, and it has come to become clear that further improved technologies are indispensable.
As described above, various compounds have so far been disclosed for organic EL device materials, and organic EL devices which provide a low operating voltage and a high luminous efficiency and which are excellent in a durability (life) and can be reduced in a voltage rise in operation are tried to be developed. However, organic EL devices in which the above performances are further more improved than ever are desired to be developed. Further, organic EL devices which are excellent as well in an ageing stability are desired to be developed. Also, organic EL device materials having an aptitude in production by the wet process are desired to be developed, and organic EL devices prepared by the above organic EL device materials are desired to be developed.